Heated till just turning red in daylight (about 550c) thrown into water and reheated again to about 250c.

Isn`t it many ways of doing quenced and tempered? check PM.

oilbath, saltwater, freshwater etc.

those 250degrees is not enough for structural chances so im assuming its for limiting further hardening into the core due to air cooling that takes place afterwards. Then assuming it got some heat remaining in the core..

Most people have heard about steel type 42CroMo4 But no one here knows what the TQ + T stands for. 42CroMo4 TQ + T

Anyone got a clue?

We also have C45 + QT

We where thinking it was Quenc Tempered but i don`t find a solid source for it.

MatsNorway,

Here in the US, the equivalent alloy steel to 42CrMo4 is 4140. I would assume that "C45 + QT" means quench and temper to Rockwell C45 hardness, which is equivalent to about 214,000 psi UTS. The heat treatment for this alloy and strength level would involve first heating the material to within its austenitizing range (1550 to 1600 degF) and holding it there for a suitable length of time. Then the material would immediately be oil quenched. Finally, the material would be reheated to approx. 725 degF (for Rc45) and held there for about 1 to 2 hours (depending upon section thickness), and then slowly cooled to room temp.

Incidentally, Rc45 hardness is about the upper limit you want to use with 4140 alloy steel.

Good guess but the C45 + QT is a separate steel. It peaks at around 650 Vickers or HRC 65 on the surface and drops down to 300 vickers 0.5mm within. lowest is 198 vickers (HV10)

Its highly likely that it is indeed Quenc tempered. With induction heater.

The 42CrMo4 TQ +T is the big mistery.

And that one is not any harder than the C45 steel it is about the same, slightly above 650 vickers but overall stronger. Im guessing it has been just Quenc tempered for 650 vickers but likely in a different matter. In a slower and milder way. Normalised perhaps? usually they then according to a sales person ad a N for normalised. C45 + N for instance.

Good guess but the C45 + QT is a separate steel. It peaks at around 650 Vickers or HRC 65 on the surface and drops down to 300 vickers 0.5mm within. lowest is 198 vickers (HV10)

Its highly likely that it is indeed Quenc tempered. With induction heater.

The 42CrMo4 TQ +T is the big mistery.

And that one is not any harder than the C45 steel it is about the same, slightly above 650 vickers but overall stronger. Im guessing it has been just Quenc tempered for 650 vickers but likely in a different matter. In a slower and milder way. Normalised perhaps? usually they then according to a sales person ad a N for normalised. C45 + N for instance.

MatsNorway,

I did a quick search and the equivalent US grade to C45 is 1045 carbon steel. 1045 carbon steel is different from 4140 alloy steel. While 1045 can also be quenched and tempered to improve mechanical properties, it would not give useful service at strength levels above about 120 ksi UTS. The reason carbon steels like 1045 are not used at higher strength levels similar to those used with alloy steels like 4140 is due to a lack of "toughness". The toughness is provided by 4140's alloying elements like chromium and molybdenum.

Your description of a part with a very hard (HRc 65) outer surface and a softer core does indeed sound like it was induction or flame hardened. Induction or flame hardening requires steel with higher levels of carbon, such as 1045 (.45% carbon). The induction hardening process only locally heats the outer surface of the part before it is quenched, but there is no subsequent tempering required since the core is mostly unaffected.